RVHS_H2_CHEM_P3_Ans Prelim

River Valley High School 9647/03/PRELIM II/16 [Turn over 2016 Preliminary Examination II RIVER VALLEY HIGH SCHOOL YEAR 6 PRELIMINARY EXAMINATION II CANDIDATE NAME CLASS 6 CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9647/03 Paper 3 Free Response 21 September 2016 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Cover Page Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class, centre number and index number on all the work you hand in. Write in dark blue or black pen on both sides of paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together, with the cover page on top.

2 River Valley High School 9647/03/PRELIM II/16 2016 Preliminary Examination II Answer any four questions. 1 Many compounds of chlorine are manufactured from brine, NaC l(aq). The electrolysis of brine produces Cl2(g) and NaOH(aq). In some industrial electrolytic cells, these two substances are allowed to r eact further. The products formed in this second reaction depends on the operating conditions used. (a) Write balanced equations for the reaction between Cl2(g) and (i) cold aqueous NaOH; [1] Cl2 + 2NaOH  NaCl + NaClO + H2O (ii) hot aqueous NaOH. [1] 3Cl2 + 6NaOH  5NaCl + NaClO3 + 3H2O (b) Chlorine dioxide, ClO2, is used in the treatment of water. It is formed from ClO3− ions in an acidic solution. ClO3− + 2H+ + e− ⇌ ClO2 + H2O (i) Draw the dot-and-cross diagrams of ClO2 and H2O2. You may assume that there is no dative bond in either compound. [2] (ii) Construct the overall equation for the reaction of C lO3– ions with H2O2 in acidic solution. [1] 2ClO3 + 2H+ + H2O2  2ClO2 + 2H2O + O2 (iii) What is the role of H2O2 in the reaction? [1] Reducing agent

3 River Valley High School 9647/03/PRELIM II/16 [Turn over 2016 Preliminary Examination II (c) Chlorine can also be used to disinfect water. When chlorine is added to water, it produces hypochlorous acid, HOCl. Hypochlorous acid is a weak acid that dissociates into hypochlorite ions , OC l−, according to the following equation. HOCl(aq) ⇌ H+(aq) + OCl−(aq) Free chlorine refers to the total chlorine content in HOCl and OCl−. The dissociation curve below illustrates the ratio of hypochlorous acid to hypochlorite at different pH values. [Source: www.hach.com] (i) Determine the pKa of hypochlorous acid [1] pH = pKa when [HA] = [A